The present invention relates to a speech synthesizer based on speech analysis and synthesis of a linear predictive coding techniques represented by PARCOR (Partial Autocorrelation) technique.
In a speech synthesizer, the synthesizing parameters necessary for synthesizing a speech in each frame are: amplitude; pitch; repeat cycle; discrimination between voiced sound and unvoiced sound; PARCOR coefficient; etc. For smoothing the sequence of the synthesizing parameters between frames, the interpolation process is executed to obtain an excellent synthesizing sound quality as disclosed in Japanese Patent Application No. Sho 56-11871 (11871/81).
A computing portion which produces a synthesized speech using the synthesizing parameters comprises a digital filter portion. If the computing data of a former frame remains within the digital filter portion in case it starts a computation, there would be a bad influence on the coming computation. More specifically, when the output from the digital filter portion is listened to as a speech through a D-A converter, the expected speech is not synthesized but instead noisy sounds which painfully affect the listener's ears are produced. Therefore, it is necessary to initialize the digital filter at the start of each frame.
By this initialization, a new frame computation which is not affected by the former frame data starts.
The "interpolation process" means that the synthesizing parameters of the former frame approach the synthesizing parameters of the latter frame in accordance with a change with the passage of time, when voiced sound frames are repeated. A smooth sequence of speech can be realized by this interpolation. In a pitch synchronous synthesizer made up of frames based on pitch period as the speech synthesizer according to the present invention, however, the sequences of the neighboring frames are sometimes unnatural only by frame initialization which resets a delay circuit at the start of each frame. Accordingly, "words" or "sentences" are unnatural and painfully the ears of the listener.